I just got my new Tor II Mk4 amp and have about 20 hours on it and it seems to have developed a hum in the right channel that can be heard from my listening position when the source is quite. I am brand new to tube amps, so the only thing that I have attempted at this point, besides switching positions of the impedance switch, reconstructive feedback switch and bass control switch, is to dis-connect the source cables from the amp so that the speakers are connected and the power is connected with no source connected. Doing this did not change the level of the hum. There is a very slight hum in the left channel, but the right channel is much louder. The hum does change and gets louder as the volume is increased.

I followed the installation instructions and it seemed that there was no difference in the channels to begin with.